Swept frequency test systems commonly comprise a sweeping signal generator, usually known as a sweeper, which produces an output signal having a frequency that varies linearly with time between predetermined limits. The output of the sweeper is passed through a device under test and is detected by a detection circuit, such as a network analyzer, spectrum analyzer or the like. The output of the detection circuit is usually displayed on a cathode ray tube (CRT) display as a function of the frequency of the sweeper. The system thus produces a visual indication of the frequency response of the device under test.
It is usually desirable to know the frequency of certain points on the frequency response display more accurately than can be determined from the horizontal axis of the CRT display. In the past it has been common to provide frequency markers on the CRT by various means. One commonly used marker is the so-called birdie which is produced by mixing the sweeper output with a reference signal to produce a signal having a frequency equal to the difference between the frequencies of the sweeper output and the reference frequency signal. This output signal is usually passed through a low pass filter and then is applied to the vertical input of the CRT display to modulate the display and thereby give an indication which shows the point in the display at which the two signals are equal in frequency. This system has the disadvantage that the rapid undulations of the birdie in the marker are often obscured by variations in the display due to the frequency response characteristics of the device under test. Furthermore, the size of the birdie depends upon not only the bandwidth of the low pass filter but on the signal strength of the two signals combined to produce the birdie.
Various attempts have been made to produce markers by detecting amplitude changes when a birdie is produced and modulating the vertical axis of the CRT display with a pulse when the birdie is detected. This likewise has the disadvantage that the amplitude detection is dependent on the strength of the signals combined to make the birdie as well as the bandwidth of the low pass filter and thus may not produce a reliable marker under all circumstances.